Don't reuse the source mbuf in tcp_respond() if it is not writable.

Don't reuse the source mbuf in tcp_respond() if it is not writable.

Not all mbufs passed up from device drivers are M_WRITABLE().  In
particular, the Chelsio T4/T5 driver uses a feature called "buffer packing"
to receive multiple frames in a single receive buffer.  The mbufs for
these frames all share the same external storage so are treated as
read-only by the rest of the stack when multiple frames are in flight.
Previously tcp_respond() would blindly overwrite read-only mbufs when
INVARIANTS was disabled or panic with an assertion failure if INVARIANTS
was enabled.  Note that the new case is a bit of a mix of the two other
cases in tcp_respond().  The TCP and IP headers must be copied explicitly
into the new mbuf instead of being inherited (similar to the m == NULL
case), but the addresses and ports must be swapped in the reply (similar
to the m != NULL case).

Reviewed by:	glebius